Automobiles, trucks and other internal combustion engine powered vehicles waste a significant amount of fuel and generate unnecessary exhaust when temporarily stopped for traffic signals or other reasons (i.e., pick-ups, drop-offs or deliveries). Such waste may be reduced by turning the engine off during these stopped periods and subsequently restarting the engine when travel is to be resumed. While this stop-restart operation may be performed manually, the process may be effectively automated.
In one approach to such automated stop-restart operation, an on-board computer may be used to detect that the vehicle has been brought to a stop for a pre-defined short period of time, and issue commands causing the engine to shut down. When the driver desires to resume travel, the accelerator is depressed and the computer automatically commands the engine to restart.
Typically, restarting the engine places a high current demand on the electrical system. This high demand causes an increased voltage drop across the vehicle's electrical system wiring harness that may disturb the performance of the lights, entertainment systems, and other accessories powered by the electrical system. This is not only a potential annoyance for the driver and passengers, but may also impact the operation and safety of the vehicle.
An approach to remedying the problems caused by high current demands associated with restarting the engine is to employ a second battery which can be switched into parallel with the primary battery when the engine is restarted. U.S. Pat. No. 5,204,610, entitled LONG LIVED DUAL BATTERY WITH AUTOMATIC LATCHING CIRCUIT, to Pierson et al., discloses a latching automatic dual battery switch that employs a comparator to determine that a load condition requiring engagement of a secondary battery exists. In accordance with the teachings, when the voltage of the primary battery voltage drops below a predetermined threshold or the current drawn from the primary battery exceeds a predetermined threshold, the battery switch automatically engages a secondary battery. This approach suffers in that a drop in voltage must occur before the switch is activated. The voltage drop can also affect the performance of other on-board electrical system components. A second patent, U.S. Pat. No. 5,316,868 entitled. DUAL BATTERY SWITCH CIRCUIT, to Dougherty et al., discloses a two battery system comprising a controller that couples the secondary battery to the primary battery with respect to the load when an attempt is made to apply energy to the load and the temperature is below a predetermined level. Here again, an undesirable voltage drop transient can occur. Each of these inventions further suffers from the need to provide separate charging and maintenance facilities for each battery.
The inventors have identified an automatic stop-restart (ASR) system that satisfies the electrical requirements of restarting the engine while overcoming the voltage drop problem and further allows a single battery charger to service all of the batteries. In accordance with the ASR system architecture, the vehicle is equipped with two batteries. A first battery is connected to supply the heavy current load required to restart the engine after being stopped. A second battery is connected to supply power to the remaining (i.e., non-starting) electrical system loads. While the vehicle is moving or stopped, both batteries are connected in parallel, by a switch network, so that they may both supply power to the entire system and be charged by the vehicle's charging system.
When the vehicle is brought to a stop, the engine is automatically shut down. When the operator wishes to resume movement, the driver depresses the accelerator and the ASR system is activated. The ASR system disconnects the batteries from each other. The first battery can then exclusively supply power to the engine starter system in isolation from the second battery and its associated harness. The first battery's harness and its voltage drop during this brief heavy electrical load will not affect the operation of the components operating from the second battery's harness.
The switch network used to connect and disconnect the two batteries may comprise a mechanical switch, such as a relay or solenoid. There are several disadvantages to the use of the mechanical switch approach including performance degradation over life, reliability, and the generation of unacceptably high levels of electrical and radio frequency interference. The switching of high currents and possibly inductive loads may result in damage to the switches electrical contacts. Such damage can lead to changes in the conduction properties of the switch and eventually to failure. With respect to interference, the almost instantaneous opening and closing of the contacts may result in high voltage impulses and high current surges. The waveform of these impulses and surges may have very fast rise and fall times which contain high frequency spectral components that can radiate or be conducted from the switch contacts. These radiated or conducted spectral components have the potential to disturb other vehicular and passenger systems.
In a preferred embodiment, the switch network used to connect and disconnect the two batteries may comprise an electronic switch network. Among the important characteristics that must be satisfied for this application by an electronic switch network are high current carrying capability, low on-resistance (i.e., connection between batteries), high off-resistance, low power consumption by the switch network, and low interference generation. With respect to repeatability of contact and reliability, electronic switch networks can greatly outperform mechanical implementations. Electronic switches can also be designed to minimize radiated and conducted electrical and radio frequency interference. In contrast to mechanical switches, the electronic network permits the turn-on and turn-off switching waveforms to be independently shaped to optimize the tradeoff between fast switching times and the generation of high frequency spectral components.